Some people complain about the quality of the FS5 UHD image. Here's a tasty example with a bit of F5 XAVC-I too.
I still do not regret the purchase because the FS5 is fine for what it does. On the other hand, it is quite questionable why companies like Sony even dare to release something that is so unusable under certain circumstances.
I also have to point that I do not think it's true macroblocking, I think it's just badly processed noise. Maybe there's a chance that Sony will develop the codec into something better.